Breast cancer accounts for 18% of all cancers in women, making it the foremost cause of cancer-related deaths in women (McPherson K et al (2000) BMJ 321(7261):624-8). Currently, routine mammography is the most commonly used method for early detection of breast cancer (Smith R A et al (2012) Oncology 26(5):471-5, 479-81, 485-6). Therefore, early diagnosis and treatment of breast cancer could play a monumental role in reducing deaths (Misek D E and Kim E H (2011) Int J Proteomics 2011:343582). Most of the drugs available for the treatment of breast cancers target growth factor and endocrine receptors, particularly the endocrine (estrogen; ER) or growth factor ((ErbB-1, ErbB-2 [human epidermal growth factor receptor 2; HER2], ErbB-3 and ErbB-4) receptors for therapy (Normanno N et al (2009) Endocr Relat Cancer 16(3):675-702).
However, emerging resistance to endocrine drugs and therapies targeted against HER2 receptors have created a dire need for identification of molecular targets that are non-receptor based and directly involved in the proliferation of the cancer cells (Normanno N et al (2005) Endocr Relat Cancer 12(4):721-47; Normanno N et al (2009) Endocr Relat Cancer 16(3):675-702). Triple Negative breast cancer (TNBC) is known to be the most aggressive of breast cancers that can metastasis beyond the breast and are more likely to recur after treatment. Tumors and cells of this subtype of breast cancer lack the estrogen, progesterone as well as the human epidermal growth factor receptor 2 and hence will not respond to the traditional therapies. Although, estrogen positive and HER2 over-expressed breast cancers have relatively good target-based agents for treatment, Triple Negative Breast cancer (TNBC) will not respond to these therapies since it lacks all these receptors. There is therefore a huge void for therapies for patients with triple-negative breast cancer (endocrine and growth receptor negative). Hence, the discovery of non receptor based target therapies that may be universally applicable to all sub-types of breast cancers is of paramount importance.
DNA replication is one of the most remarkable and challenging steps in the cell cycle and requires the collaboration of a formidable number of proteins. In eukaryotes, several accessory proteins such as Replication Factor C (RFC) and Proliferating Cell Nuclear Antigen (PCNA), confer speed and high processivity to the replicative polymerases, DNA polymerases δ (Pol δ) and ε. The RFC loads PCNA onto DNA and consists of five subunits, RFC140, RFC40, RFC38, RFC37 and RFC36 (Gupte R S et al (2005) Cell Cycle 4(2): 323-329). Its assembly commits the cell to DNA replication and is involved in many DNA transactions such as DNA damage checkpoint response, maintenance of genomic stability and regulation of sister chromatid cohesion in mitosis as well as in meiosis (Majka J and Burgers P M (2004) Prog Nucleic Acid Res Mol Bio 178: 227-260; Petronczki M et al (2004) J Cell Sci 117(Pt 16): 3547-3559).
Amongst all the RFC subunits, only the second subunit, RFC40/RFC2 can independently unload PCNA and inhibit DNA Pol δ activity (Cai J et al (1997) J Biol Chem 272(30):18974-81; Pan Z Q et al (1993) Proc Natl Acad Sci USA 90(1):6-10). It has been recently discovered that RFC40 is required for accurate chromosomal segregation and completion of cell division after mitosis in proliferating neonatal rat cardiac myocytes, suggesting a role for RFC40 in mitosis and cytokinesis (Ata H et al (2012) PLoS One 7(6):e39009). Additionally, it was also observed that inhibition of endogenous RFC40 in proliferating neonatal rat cardiac myocytes causes cell death (Ata H et al (2012) PLoS One 7(6):e39009). Consistently, it has been demonstrated that deletion of RFC40 gene is embryonically lethal in yeast (Cullmann G et al (1995) Mol Cell Bio 115(9):4661-71). Also, halo-insufficiency of RFC40 causes growth retardation in Williams-Beurner syndrome (Peoples R et al (1996) Am J Hum Genet 58:1370-3). Taken together these findings suggest that RFC40 is essential for cell proliferation.
Interaction between the second subunit of the Replication Factor C, RFC40, and the regulatory subunit of Protein Kinase A, R1α, has been identified using yeast two-hybrid screening (Gupte R S et al (2005) Cell Cycle 4(2): 323-329). This complex has been shown to be essential for cell survival and that R1α functions as a nuclear transport protein for RFC40 via its non-conventional nuclear localization sequence (NLS) (Gupte R et al (2005) Cancer Biology and Therapy 4(4):429-437). Moreover, deletions in the RFC40 binding region on R1α, or either deletion or mutations of the non-conventional NLS of R1α, leads to G1 arrest, suggesting that the R1α-RFC40 complex is transported to the nucleus at the G1/S transition. Additionally, elevated intracellular cAMP levels exert transcriptional/post-transcriptional effects on mRNA levels and a translation effect on the protein expressions of both RFC40 and R1α, thereby increasing the amount of the R1α-RFC40 complex formation and hence promoting the nuclear transport of RFC40 by R1α (Gupte R et al (2006) Exper Cell Res 312:796-806).
Currently there clearly is a need for molecular targets that are non-receptor based for use and application towards the development of drug therapies for breast cancers lacking endocrine and growth receptors such as triple negative breast cancers (TNBC) and for therapies which are endocrine and growth factor receptor independent and therefore applicable to all or most forms of breast cancer. The present invention provides a novel independent marker and target for cancer diagnosis and therapy, particularly including breast cancer.
The citation of references herein shall not be construed as an admission that such is prior art to the present invention.